Coating compositions, such as powder coating compositions, often include colorant and/or filler particles to impart color and/or performance properties in the resulting coating. Pigment particles tend to have a strong affinity for each other and, unless separated, tend to clump together to form agglomerates. Therefore, these agglomerates are often dispersed in a resinous grind vehicle and, optionally, dispersants by milling or grinding using high shear techniques to break up the agglomerates. If nano-sized pigment particles are desired, further milling is often required to obtain the desired particle size.
Pigments and fillers usually consist of solid crystalline particles ranging in diameter from about 0.02 to 2 microns (i.e., 20 to 2000 nanometers). Agglomeration is a serious problem for nano-sized particle pigments and filler materials (such as carbon black) in particular because these nanoparticles have a relatively large surface area. Thus, acceptable dispersion of such nanoparticles often requires an inordinate amount of resinous grind vehicle and/or dispersant to effect de-agglomeration and to prevent subsequent re-agglomeration of the nanoparticles.
The presence of such high levels of resinous grind vehicles and dispersants, however, in the final coating composition can be detrimental to the resultant coating. For example, high levels of dispersants have been known to contribute to water sensitivity of the resultant coating. Also, some resinous grind vehicles, for example, acrylic grind vehicles, can negatively impact coating performance properties such as chip resistance and flexibility.
Powder coatings compositions for use in coating various types of substrates are often desired. Such coating compositions can greatly reduce, or even eliminate, the use of organic solvents that are often used in liquid coating compositions. When a powder coating composition is cured by heating, little if any volatile material is given off to the surrounding environment. This is a significant advantage over liquid coating compositions in which organic solvent is volatized into the surrounding atmosphere when the coating composition is cured by heating.
It would also be desirable to provide an aqueous dispersion of resin-enclosed particles, wherein re-agglomeration of the particles is minimized, and which may be suitable for use in preparing powder coating compositions.